LARGE BIPOLARONS AND SUPERCONDUCTIVITY

Superconductivity has long been speculated to result from charge carri ers paired as mobile charged bosons. Although the pairing of carriers as small (single-site) bipolarons is known, small bipolarons readily l ocalize. By contrast, large (multi-site) bipolarons, in analogy with l arge polarons, should be mobile. It is shown that large bipolarons can form in solids with very displaceable ions, e.g., many oxides. Large- polaronic (but not small-polaronic) carriers produce absorption spectr a like the carrier-induced absorptions observed in cuprates. Redistrib ution of the self-trapped carriers of large bipolarons among sites of carriers' molecular orbitals in response to atomic motions lowers phon on frequencies. The dependence of the phonon zero-point energy on the spatial distribution of large bipolarons produces a phonon-mediated at traction between them. This dynamic quantum-mechanical attraction fost ers the condensation of large bipolarons into a liquid. Superconductiv ity can result when the large-bipolarons' groundstate remains liquid r ather than solidifying.